Tuned radio signal circuit



June l0, 1941. R, LI HARVEY .2,345,349

TUNED RADIO SIGNAL CIRCUIT Filed May 5l, 1939 Patenieci l@ lfill RobertL. Harvey, Qalilyn, N. ll., assigner to adio Corporation of America, acorporation of melaware Application May 3l, w33, Scial No. 2i

il Claims. (Cl. Titi-37D The pre-sent invention relates to tuned radiosine-,ll variations in the strength of the applied signal circuits, andmore particularly to signal signal or in response to variations in theapplied circuits of the type having ferro-magneiic cr signal voltage.

inagne-tite core inductance tuning. It is also a further object of thepresent in- In conventional circuit design with inductance 5 c ltion toprovide an improved tuned signal contuning of the type above referredto, the change veying circuit, the resonant frequency of which inpermeability is made relatively small and usu be var ed in response toVariations in the ally is not measurable because of the low flux strerth of the applied signal or in response to density and relatively largeVolume of the cores variations in the app-lied signal voltage whenordinarily provided. It has been found, however, lo such voltagelariations are relatively wide and that with magnetite core tuninginductances, the the applied signal is relatively strong. permeabilitymay be varied appreciably as the lo providing a circuit embodying theinvenflux density in the core is increased to a relatively tion, atuning inductan'ce or inductive reactance .high value. of `theferro-magnetic core type in which the core The flux density may beincreased bythe presl5 material is of niagnetite is arranged to operateence of strong signals as in an IF amplifier, to inwith a relativelyhigh linx density for high frecrease the signal current flow through thcinquency signal circuits. With magnetite cores in ductance, or the coreVolume may be decreased the tuning inductances, this has been found toto a relativelysmall percentage of anormal core f 'case the effectivepermeability and to cause volume. It has been found that as a result ofa 2O an i crec.se in the inductan-ce value and a correlatively higherflux density than has heretofore espendin heen used for high frequencysignal circuits, .it of the tun d circuits. is poesie-lesto shift orvary the resonant frequency lt is, therefore, a still further object ofthe of a tuned circuit embodying such an inductance, present inven ionto provide an improved tuning over a relatively Wide frequency 'rangeWith a y25 i .l element for high frequency radio sigrelatively narrowrange of variation in the nal circuits wherein the eifectivepermeability strength of the applied signal or signal voltage, of aferromiagnetic core may be varied in the because of the change inpermeability, and that, presence of strong signals and with a relativelyfor maximum change of flux density with relalow variation in signalstrength or voltage. tively small voltage change, a lovv L/C ratio in 30In. a tuned radio signal cir-cui't such as the cirthe tuned circuit isnecessary. cuit of an intermediate frequency amplifier, it It is,therefore, an object of the present inhas been found that thepermeability variation of vention to provide an improved tuned radiosiga ferro-magnetic core tuning inductance may be nal circuit embodyinga tuning inductance or increased by making the L/C ratio of the circuitinductive reactance device of the magnetite core 35 loi-v, that bymaking the inductance relatively type, wherein a higher than normal fluxdensity and the associated shunt tuning capacity relfor radio signal.circuits is provided in the core atively high in value, thereby toprovide an inin response to an applied signal vol-tage to cause creasein sig al current flow since tbe flux in the frequency response of thetuned circuit to be l the core depends upon the factor IZL. The flux,varied in response to variations in applied signal M theefore, isproportional to I2. It is, therefore, strength or voltage. an object ofthe present invention to provide a It is also an ooiect of the presentinvention to permeability tunable inductance for high freprovideimproved radio signal circuit of the quent-y tuned radio circuitsaranged to take adinagnetite core inductance tuned type which is vantageof the increased current available in a responsive to variations insignal strength to l5 circuit having a lovv L/C ratio to increase theefcause a corresponding variations in pernielabillect of frequencyvariation with variations in ity and frequency response, whereby ythecircuit permeability. is adapted for use in compensating circuits for Itis also an obje-ct of the present invention to high frequencyamplifiers, for example, as ernprovide an improved high frequency radiosignal ployed in the and IF circuits of a superlo circuit of theferro-magnetic tuned type comheterodyne receiver, and also in connectionwith ng a inductance having a magnetite AVC and AFC circuits for radioreceiving app'acore and which, in the presence of strong signals, ratosand the like. responds to variations in signal strength or volt- Theinvention is particularly adapted for use age to provide a change in thepermeability of in connection with the tuned signal conveying 5;3 thecore the inductance value thereof,and circuits of radio and other highfrequency apirnproved control circuit, such as an AVC circuit paratus,has for its furet-her object to provide for radio receiving apparatusand the like, ernan improved tuned signal conveying circuit, the ingsuch tuning inductance or inductances.

resonant frequency of which may be varied over a S, ice an increase insignal strength or voltage relatively Wide range in response torelatively J in a tuned circuit embodying the invention provides adecrease in resonance frequency, the circuit may be utilized tocompensate for variations in capacity on the circuit caused byvariations in controlling potentials on associated amplifier tubes such,for `example, as variation in capacity provided by an associated controlgrid in response to variations in AVC potential thereon to maintain theresonance frequency substantially constant.

The invention will, however, be better understood from the followingdescription when considered in connection with the accompanying drawing,and its scope will be pointed out in the appended claims.

In the drawing,

Figure l is a schematic circuit diagram of a tuned radio signal circuitembodying the invention;

Figure 2 is a graph showin-g curves representing operatingcharacteristics of the circuit of Fig. 1 under diifering conditions ofadjustment;

Figure 3 is a circuit diagram of an intermediate frequency amplifier andsecond detector for a superheterodyne radio receiving system alsoembodying the invention; and

Figure 4 is a graph showing curves illustrating an operatingcharacteristic of the circuit of Fig. 3.

Referring to Fig. 1, a tuned radio signal circuit 5 is provided with atuning inductance 6 and a shunt tuning capacitor 1 for said inductance.A signal input circuit 8 is coupled through a winding 9 with theindurctance 5. This represents any suitable signal conveying circuitadapted to be tuned to resonance at a predetermined frequency.

In accordance with the invention, the inductance 6 is of theferro-magnetic core type having a core I!) of magnetite of such volumeor crosssection that the flux density therein is relatively high.

The magnetic circuit of the core I@ is preferably closed through asuitable path provided by magnetic core material providing a relativelylarge volume or Ycross-sectional area. In. the present example, the endsof the core lil engage the inner surfaces of a ring Il of magneticmaterial having a large cross-sectional area Vwith respect to that ofthe core Ill.

By this arrangement, the core volume or crosssection under the windingis decreased. The signal intensity applied to the input circuit 3 andthe voltage E in the tuned circuit 5 may be relatively high as in anintermediate frequency amplifier, with the result that the ux density inthe core I El is increased to such a point that the permeability of thecore is affected and, as hereinfbefore pointed out, the effect is toincrease the permeability with increase in the iiuX density so that itis possible to shift the resonant frequency of the tuned circuit 5 overan appreciable frequency range by variation in the signal intensity orvoltage E across the tuned circuit.

As shown in Fig. 2 by the curve l2, it has been found to be possible toshift the resonant frequency of the tuned circuit 5 from 450 kc., for

example; to 430 kc. with an RF voltage changeof Afrom zero tosubstantially ten volts. This is with a low L/C ratio in the tuned|circuit '5. With a higher L/C ratio, the response curve may be asindicated at I3 in Fig. 2, requiring a wider variation of voltage Eacross the tuned circuit to obtain the same shift of the resonancepoint.

' The value of the capacitor 1 or the value of C in the :circuit 5 was400 mmf. for obtaining the curve I3, while for obtaining the curve i2, acapacity of 4000 mmf. was employed across the tuning inductance. rIhelower L/C ratio provides a higher current through and a lower voltageacross the tuning inductance. With a higher value of tuning capacity, itis obvious that for the same frequency or frequency range the value ofthe inductance B and the number of turns on the winding must be lower.However, it has been found that with a lower number of turns, the fluxthrough the core is increased appreciably because of the fact .that theflux varies as the square of the icurrent and only as .the first powerof the inductance. A

The ratio of the reactance of the inductance 6 to its high frequencyresistance, or the Q of the coil, is reduced with increase in thestrength of thesignal applied through the circuit toV the circuit 5,because the high frequency resistance increases at a higher rate thanthe inductance or the effective permeability, and this results in adecrease in the gain of any amplifier stage in which the circuit isconnected, which is a desirable feature since it reduces the gain forstrong signals and permits increased gain automatically for weaksignals.

This is for the reason that, in the present system, the core of theinductance operates with high frequency currents far below the knee ofthe permeablity-magnetomotive force or saturation characteristic rcurve,but still relatively higher than has heretofore been used in suchsystems. This causes the permeability to rise with increased RF voltageat E. The relatively high ux density herein referre-d to is a fluxdensity which is high for that ordinarily obtainalble in high frequencysignal circuits and should not be confused with that used in power andaudio frequency circuits. On a usual saturation curve, for example,v apoint above the upper bend or knee is that which is ordinarilyconsidered to represent a high fiux density for power and audiofrequency circuits, while a point on or above the lower bend of the samecurve l would indicate a high radio frequency ux density. The flexing ofthe curve is opposite for the two cases, and it will therefore be seenthat the tangent to the curve more nearly approaches the vertical forthe radio frequency circuits and, accordingly, Yin this region, as theRF voltage and current increases, the permeability increases, since thetangent to the curve becomes further more nearly vertical.

As will be seen from the icurves of Fig. 2, the voltage E required tochange the resonant point or tuning o-f the circuit 5 through arelatively wide frequency range may be varied over a relatively widerange or a relatively narrow range, depending upon the L/C ratio 0f thetuning elements in the circuit. This permits considerable latitude inthe design of the circuit.

In any case, however, the range of frequency variation and the Variationof the Q of the coil is relatively high only when the flux density isincreased to a relatively high value for such high frequency lcircuitsas hereinbefore noted, and this is accomplished not only by providing aninductance winding having a magnetite core of relatively low volume orcross-section whereby the flux density for high frequency circuits ishigh per unit volume of the core, but is also made possible at a lowerinput voltage E, by providing the tuned circuit with a low L/C ratio.

In any tuned circuit, it is unneccessary to depart from the desirablefeatures of magnetite core tuning to obtain a shifting of the resonantpoint or a frequency variation in the tuned circuit, since to obtainthis characteristic, it is merely necessary to decrease the length anddiameter of the coil and core asseinhly in such a manner that the iiuxdensity per unit of voluine of the core is increased, and for a maximumchange in frequency with low voltage change, a 10W L/C ratio is used inthe tuned circuit.

Referring new to 3, an intermediate ire-- quency amplier circuit isshown in which a tube iii is provided as the last intermediate frequencyamplifier stage precening a second detector tube in the signal channelof the receiver, The detector is of the diode rectine type coinprisint,Ta pair of diode electrodes il connected to the secondary it oi IEcoupling transformer is which is connected in series with a couplingcoil 2t and is tuned by a shunt adjustable capacitor i.

The output circuit of the iF ampliiier tube i5 is connected with thetuned primary winding also provided with a shunt tuning capacitor 2li sothat the two tuned circuits rnay respond to the saine fred ncy, such as450 kc., for example.

The secondary circuit liB--Il-Zl is connected at its. low potential endthrough a diode output resistor ii to the cathode Audio frequency signaloutput is derived` from the resistor 2d through an output leasj 2i',connected in the present example to an ain grid 2S in the same envelopewith the diode detector. Audio irequency signals for furtheramplification and utilization are derived from the output anode il@ asusociated with the grid is, through the output circuit 3S.

The Ei tuned input circui tube i5 is provided with a comprising a tuninginductance 35i and shunt tunii e capacitor 3,; therefor forming thetuned secondary cir-cui* of an L? transformer 3l ed primary of which isindicated at secondary is tuned by the shunt cap and also by a core Si?of magnetic m the ein in f1 cross-sect to normal. s; sunpiied throughthe channel, the voltage the circuit 35i- 36 causes the flux density thecore to he relatively high for high fre uel :y circuits although farbelow the upper bend the usual saturation curve for inductanoe elementsin audio frequency and low frequency power circuits, thereby to decreasethe frequency of resor ce ci the input circuit by an amount lest sucient to overcome the eiect on the circuit c at or control grid of tireen pliiier connected thereto and y with a source oi AVC As is well biasprovided in Ai strength in an AVC l such as inagnetite arranged -iysuitable manner and being of such area or volume that, in response rilnotential known, -e response to systeni cans-s to be door i Vrequencyresponse to he shifted toward a higher resonant frequency. This tendencyis overcome by the present arrangement.

To take oi the variable permeability in response to variations in signalstrength to the tuning' ci a resonant circuit having inductanceferro-magnetic core tuning` means, a tuned cir comprising a tuninginductance fit and a 'nt capacitor lil, is associated with the couplingwii in such a manner that, as indicated in the drawing, the latterWinding is As ereinleeiore described, the increase in perineao itycauses an increase in inductance of the windings tti and tunes thecircuit to a lower frequency, to increase this effect, the magneticcircuit is preferably closed, as in the arrangement i, furthermore, theratio of the inductance it to the capacity at that is, the C ratio, isrrwe low. This provides a relatively high current and low inductance inthe winding for a given frequency.

With an intermediate frequency circuit tuned to kc., the circuit may betuned to 486 kc., for example, with weak signal input. As the appliedsional strength is increased, the 480 kc, circuit [is will shirt inresonant frequency to a lower frequency, approaching the intermediatefrequency 45) irc. as a liinit, thus increasing the coupling to thesecondary and causing the output voltage from the .,'ied circuiu i5 toincrease at a higher rate than the signal.

A signal of the diode type indicated at having a output resistor 5l maybe connected across the tuned circuit to provide controlling potentialsfor the AVC circuit (il, the connection being made indicated, through asuitable i'lter ccinpri a series resistor 52 and a shunt niter.capacitor This causes a better AVC characteristic than is normai yrevidecL as indicate the curves o; 4, in which the curve is a responsecurve for a norrnal AVC connect-i n, W creas the curve "it representsthe AVC control cn racteristic of the circuit of Fig. 3, clearly showingthat the AVC characteristic is much flatter the prese. t system.

I claim as my invention:

l. In a tuned radio signal circuit, an inductive reactance responsive tothe flow of relatively strong signal currents therethrough to vary thetuning oi said circuit from higher to a lov/er resonant frequency,reacts-ace comprising an inductive winding a core element thereior soproportioned that the and perlneuhility :i crease with an increase insignal potential across said circuit and resulting increase in signalcurrent through said winding.

2. In a tuned radio signal circuit, an inductive reactance responsive torelatively strong signal current the r'ethrougli to vary the tuning ofsaid circ from a higher to a lower resonant frequency with rease insignal potential across said circuit, inductive reactance comprising aninductance win and a ferro-magnetic core element therein su Vt to arelatively high flux de sity in response to si'f'nal current of apredernlined value through. said reactance and an increase in thepermeability oi said core element in res ense to an increase in fluxdensity effective to e said lewe ing of the resonant frequency cuit,said inductive reactance comprising an inductive windinghaving aferro-magnetic core element providing a relatively high flux density inresponse to the iicw of signal currents in the winding and an increasein the permeability of said core element.

4. In a tuned radio signal circuit, an inductive reactance responsive torelatively strong signal currents therethrough to vary the tuning ofsaid circuit from a higher to a lower resonant frequency with increasein signal potential across said circuit, said inductive reactancecomprising an inductive winding having a. ferro-magnetic core elementproviding a relatively high flux density in response to the flow ofsignal currents in the winding and an increase in the permeability ofsaid core element, and means providing a shunt tuning capacitor for saidinductive windingY of a capacity value such that the L/C ratio of saidcircuit is relatively low.

5. In a tuned radio signal circuit, the combination of a tuninginductance and a shunt tuning capacitor therefor providing a circuit forthe rlow of signal currents through said inductance, a ferro-magneticcore element for said inductance of relatively small volume providing arelatively high flux density therein in response to an increase insignal currenm through said inductance and an increase in thepermeability,

whereby the frequency response of said circuit is decreased withincreased signal strength, and the L/C ratio of said circuit beingrelatively low for maximum frequency change in the resonance of saidcircuit in response to a relatively small signal voltage change in saidcircuit.

6. In a radio signal circuit, means for varying the tuning of saidcircuit in response to variations in a signal voltage applied thereto,said means comprising an inductance, a tuning capacitor in shunttherewith, a magnetite core element for said inductance providing arelatively high flux density in the core in response to signal currentiiow in said inductance, and means for applying radio signals to saidcircuit to cause the flux and the permeability of said core to increasewith increased signal strength, thereby to increase the value of saidinductance and to lower the resonance frequency of said circuit.

7. In a tuned radio signal conveying circuit, means responsive tovariations in signal voltage across said circuit for varying theresonance frequency of said circuit, said means comprising an inductivetuning reactance in said circuit, a ferromagnetic core for saidreactance providing an increase in flux density and permeability with anincrease in said voltage, and means for applying signals of apredetermined strength to said circuit, said last-named means comprisingan intermediate frequency amplifier circuit, a rectifier deviceconnected with said first-named circuit, and a control circuit forderiving an automatic volume control potential therefrom.

8. In a tuned radio signal conveying circuit, means responsive tovariations in signal voltage across said circuit for varying theresonance frequency of said circuit, said means comprising an inductivetuning reactance in said circuit, a ferro-magnetic core for saidreactance providing an increase in flux density and permeability with anincrease in said voltage, means for applying signals of a predeterminedstrength to said circuit, said last-named means comprising anintermediate frequency amplifier tuned circuit, a rectier deviceconnected withY said first-named circuit, and a control circuit forderiving an automatic volume control potential therefrom, saidintermediate frequency amplifier circuit being tuned to a predeterminedintermediate frequency and said first-named circuit being tuned to ahigher frequency by an amount substantially equal to a range offrequency variation of said first-named circuit corresponding to avariation of the signal voltage across said first-named circuit from aminimum to a maximum value.

9. In a radio signal conveying channel, the combination of meansproviding a tuned signal circuit, an amplifier tube having a. controlgrid connected with said tuned circuit, means for applying a biasingpotential to said control grid to vary the gain of said tube and thecapacity load provided thereby on said tuned circuit, and means in saidcircuit for compensating for variations in said capacity load tomaintain the resonance frequency of said circuit substantially constant,comprising an inductive reactance Winding in said tuned circuit and aferromagnetic core element therefor providing a relatively high fluxdensity per unit volume in response to signals through said circuitthereby to increase the permeability of said core and to decrease thefrequency response of said circuit With increased signal strength.

10. In a radio signal conveying circuit, the combination of a tunedsignal circuit comprising a tuning inductance and a shunt capacitortherefor, an amplifier tube having a control grid connected with saidcircuit, means for applying automatic volume control potentials to saidcontrol grid in response to received signals, the capacity load providedby said control grid being thereby decreased with increased signalstrength tending to increase the resonance frequency of said circuit,and means for compensating for said increase in resonance frequencycomprising a magnetite core element for said inductance providing arelatively high flux density per unit volume in response to signalsthrough said tuned circuit and a corresponding increase in thepermeability and a decrease in the frequency response of said circuitsubstantially equal to the increase in frequency response tending to beeffected by volume control of said grid,

11. In a radio receiver, an automatic volume control system, comprisingin combination, a tuned signal circuit, a second tuned signal circuitcoupled therewith, said second circuit being tuned to a higher frequencythan said firstnamed circuit and comprising a tuning inductance having amagnetite core element of relatively srn'all volume within saidinductance to provide a relatively high flux density per unit volume inresponse to signals applied to said second circuit and of a value suchthat the permeability of .said core element is increased thereby by anamount suicient to decrease the frequency response of said secondcircuit with increased signal strength to substantially the resonancefrequency of the first-named circuit, a rectifier device connected withsaid secondnamed circuit, and means for deriving automatic volumecontrol potentials therefrom for said receiver.

ROBERT L. HARVEY.

